1. Field of the Invention
This invention relates to sheet transfer devices and more particularly to vacuum lifting devices and methods of using same.
2. Description of the Technical Problems
It is recognized to lift and transfer objects by engaging a surface of the object with a vacuum device. It is also recognized to provide a plurality of vacuum chambers in a single device for lifting large objects, or for simultaneously lifting a plurality of objects. U.S. Pat. No. 3,523,707 to Roth in general teaches a vacuum lifting device having a vacuum source in communication with a vacuum plenum, and a plurality of vacuum chambers each communicating with the plenum by a single orifice. Vacuum is drawn through the vacuum plenum to each chamber through their respective orifices, and a light weight freely moveable ball in each chamber is responsive to and controls the flow of ambient air therethrough. A flow of ambient air through the chamber lifts the lightweight ball from its rest position and seats it into the communicating orifice between the chamber and plenum to control the vacuum. If an object is in proper position adjacent a chamber, air flow will not result, the light weight ball will not be lifted to isolate the chamber from the plenum, and the vacuum from the plenum will act on the object to lift same. There are limitations to the above-described device, however, because a positive seal is not assured between the ball and orifice resulting in possible vacuum leakage in unused chambers i.e. chambers not adjacent the object. Further, to release objects from engagement with the device, all the vacuum in the plenum chamber is released to the ambient atmosphere. Still a further limitation is that the device, when activated, automatically applies suction to all objects adjacent to the vacuum chambers and therefore is not capable of lifting selected ones of a plurality of sheets or objects.
U.S. Pat. No. 3,591,228 to Webb teaches in general a vacuum lifting device having a vacuum source and a plurality of vacuum chambers each communicating with a plenum through a single orifice. A spring mounted valve member within each chamber has a valve stem end extending beyond the engaging surface. The valve stem opens its respective orifice when contacted with the object to be lifted. In this manner, Webb's device automatically activates only those chambers in contact with rigid objects. Although the device is acceptable for its intended purpose, it is not readily usable for lifting selected ones of a plurality of objects or sheets, because all chambers having their valve stems in contact with rigid objects are automatically activated. Further, to release objects it is necessary to release the vacuum within the plenum to the ambient atmosphere.
U.S. Pat. No. 2,783,018 to Lytle in general teaches a vacuum lifting device having a dual valve system to activate and deactivate a vacuum chamber. A first valve assembly located between a vacuum source and the chamber operates in a first position to provide vacuum into a lower portion of the chamber, below a second valve located within the chamber. When an object is adjacent the lifting surface of the chamber the vacuum in the lower portion of the chamber pulls the second valve down onto its valve seat to seal the lower portion of the chamber from ambient atmosphere to exert suction upon the object to lift same. By moving the first valve to its second position, vacuum is diverted from the lower portion of the chamber to lift the second valve from its valve seat and permit communication between the ambient atmosphere and the chamber, to release an object from engagement with the device. This device is limited, however, by its mechanical complexity.
U.S. Pat. No. 3,202,302 to Insolio and U.S. Pat. No. 3,592,334 to Fleischauer each in general teach vacuum transfer conveyors having endless moving porous belts. Sheets are urged against the belt by vacuum and transferred to a destination where they are released. These devices are limited by their inability to lift selected ones of a plurality of sheets and by their inefficient use of vacuum which results from continually pulling vacuum through areas of the porous belt which are not engaged to a sheet. It would be advantageous to have a vacuum lifting device which could selectively lift objects while avoiding the limitations discussed above.